1. Field of the Invention
This invention relates to a monitor, and method, for monitoring temperatures at various specific times and locations in a test or manufacturing process, and for comparing those temperatures with respective standards or norms for those times and locations. It is especially well suited for monitoring pertinent temperatures of critical zones of reflow ovens used in the mass soldering of the connections of electronic circuit boards and their components, or in similar processes. Although especially well suited for use in the manufacture of electronic circuit boards (hereinafter called xe2x80x9ccircuit boardsxe2x80x9d), the invention is also useful in a number of process industries, some of which are not related to electronics.
The subject matter of this invention has been recorded with the U.S. Patent and Trademark Office under the Document Disclosure Program. The following Document Disclosure Numbers have been accorded to those records:
No. 426554 on Oct. 14, 1997;
No. 427127 on Oct. 31, 1997; and
No. 431579 on Feb. 9, 1998.
Separate letters are being submitted in this application, requesting that those Disclosure Documents be permanently maintained by the Patent and Trademark Office.
2. Description of the Prior Art
A closely-related, informative item of technical literature is U.S. Pat. No. 5,739,443, entitled xe2x80x9cCarrier for a Monitoring Devicexe2x80x9d and issued on Apr. 14, 1998 to the present applicant. In order to conserve space, the entire xe2x80x9cDescription of the Prior Artxe2x80x9d set forth in columns 1, 2 and 3 of that patent is incorporated herein by reference. Columns 1 and 2 include a comprehensive introduction of the topic of xe2x80x9cthermal profilingxe2x80x9d in reflow ovens and wave soldering machines. However, it will be desirable to present more details in the paragraphs which follow, before moving on to the novel features of this invention. Temperatures, at certain times and locations in a reflow oven, are of critical importance in the mass soldering of circuit boards. On the other hand, there may be other process industries in which temperatures have comparable importance. They should not be overlooked in a study of the background of this invention.
U.S. Pat. No. 5,820,266xe2x80x94Fedak discloses a xe2x80x9cTraveling Thermocouple Method and Apparatus,xe2x80x9d which is directed to the processing of components, such as microprocessor chips, supported on a tray or plate on a driven conveyor. The disclosure relates to ways of measuring the temperature of the product during its passage through a vertical oven, having corners, without entangling the leads of the thermocouples as they are pulled through the oven. The Fedak patent does not disclose, or imply, anything like the monitor and method of the present invention.
It is an object of the invention to provide a monitor and method for economically and repeatably obtaining thermal-profile data from a process oven, whereby to detect changes in thermal process parameters, typically in the mass soldering of circuit-board assemblies.
Briefly, the monitor in accordance with this invention comprises a structural assembly of temperature-resistant materials, instrumented with thermocouples, preferably near both the top and the bottom of the assembly, and adapted for passage through a multi-zone oven, or other heat-processing enclosure, of arbitrary width, length and conveyor speed. The thermocouples of the assembly are to be connected to input terminals of a xe2x80x9cthermal profilerxe2x80x9d or other data logger which records the output voltages of the respective thermocouples at various specific times and locations in the oven. The monitor simulates, but with greater sensitivity and repeatability, the thermal response of thermocouples mounted on the top and bottom of a typical circuit board which is to be replicated in quantity. The monitor is constructed to minimize deterioration of its components and change of its response with use and the passage of time.
The method in accordance with this invention comprises the following steps:
1. Establish the proper thermal profile for a circuit board to be processed in the oven by:
(a) attaching thermocouples at critical locations on a typical assembled circuit board and connecting them to respective input terminals of a thermal profiler which is to accompany the circuit board through the oven;
(b) repeatedly passing the typical circuit board and thermal profiler through the soldering process by means of the conveyor running through the oven; and
(c) adjusting the conveyor speed and the set temperatures for respective zones of the oven until the optimum thermal profile is established for that typical circuit board; and then
2. Record a reference thermal profile by:
(d) disconnecting the thermocouples from the thermal profiler and connecting the monitor to the thermal profiler;
(e) passing the monitor and thermal profiler through the oven on the conveyor without changing the speed or set temperatures; and
(f) establishing in permanent memory the reference profile derived from each of the thermocouples of the monitor as a function of time and/or location (zone) in the oven; and later
3. When it becomes necessary or desirable to verify or reset any or all of the zone temperatures of the oven, do so by:
(g) passing the monitor and thermal profiler through the oven on the conveyor;
(h) recording and plotting the current thermal profile of the oven as a function of time and/or location (zone) in the oven;
(i) comparing the current thermal profile with the reference profile; and
(j) adjusting the set temperatures of the respective zones of the oven and repeating steps (g), (h) and (i) until the current profile substantially matches the reference profile.